JSON Artifacts: Common metadata field

[beckjake]

Describe the feature

All JSON artifacts (catalog.json, manifest.json, sources.json, and run_results.json) should have common metadata fields. Those metadata fields should contain the common metadata that all fields have. Eventually (once everything in #2671 is implemented), I'd expect it to be:

• invocation_id
• dbt version
• dbt_schema_version
• the env metadata fields
• generated_at

There should also be artifact-specific metadata fields. For example, the existing metadata field in the manifest can be added to the above fields, just for the manifest.

Depending upon the order we do this in relative to other json artifact issues, some of the fields listed above might not exist. That's ok!